I stumbled across a nice farming spot where you will not only get a nephalem valor stack but also have a non-attacking named mob. It's not shown in the video because it was recorded with my lvl 38 toon but trust me, it works.
I've uploaded a video:
And here is my guide:
First select the Quest "Act I - The Imprisioned Angel - The Cursed Hold" and teleport to the Halls of Agony Level 2.
Once you went through the Halls of Agony Level 2 you will have to make your way towards The Cursed Hold, which is straight forward and rather short. Once you enter the prison your progress is saved. You may see the mentioned mobs because they belong to an event, which is not always the case. But out of 8 tries the mobs where there about 50% of the time.
You just have to kill the Dark Berserkers first which will give you a nephalem valor buff.
Dargon will not attack as you have not started the event on every repeat.
I hit up the woods first, there are elites in both dungeons, plus the event... then I head to the cemetary dungeons for another couple stacks and then I'm good for the Warden and the Butcher. Got 60 rares today, only two were good, and I can't even use them for my class.